I had the annoying zooming thing happen to me when I had had the phone turned briefly turned off last week (something I probably haven't done more than three or four times in total in the couple of months I've had it).
No matter what I did or which image program I used to set the wallpaper, as soon as I was back to the homescreen the image was zoomed in.
I can't entirely remember how I resolved the problem, but I'm fairly certain I managed it by switching back to the default TouchWiz interface, setting the wallpaper from there, and then switch back to GoLauncher, where the wallpaper then retained it's proper size.
You didn't mention anything about using a launcher alternative though, so I imagine it's not necessarily some conflict there that caused the problem, it just turned out to be a way to solve it for me.